HR: 1340h
AN: V33C-1510    [Abstracts]
TI: Advanced fractional crystallisation and homogenization of large-volume rhyolite before the Oraefajokull 1362 AD plinian eruption, SE Iceland
AU: Selbekk, R S
EM: r.s.selbekk@nhm.uio.no
AF: Natural History Museum, Geology, Univ. of Oslo Box 1172, Blindern, Oslo, N-0318, Norway
AU: * Tronnes, R G
EM: r.g.tronnes@nhm.uio.no
AF: Natural History Museum, Geology, Univ. of Oslo Box 1172, Blindern, Oslo, N-0318, Norway
AB: In the 50 km wide Icelandic rift zones rhyolite magma is generated by partial melting of hydrated metabasaltic crust, subsiding under the weight of the growing volcanic pile. This mechanism of silicic melt formation is indicated by the basalt-rhyolite bimodality and rhyolite O-isotope composition. The low 18/16O-isotope ratios of rift zone rhyolites trace the high-latitude meteoric water component of the subsiding hydrated basalts [1]. The rhyolites of the volcanic flank zones (VFZ), however, have generally as heavy oxygen as the associated alkaline to transitional basalts and intermediate volcanics [2,3]. The minor volcanic loading of the older, thicker and stronger VFZ crust is insufficient for significant subsidence, and less pronounced basalt-rhyolite bimodality combined with other geochemical features support silicic melt generation by fractional crystallization. An extreme case in Icelandic, as well as global, perspective is the rhyolite magma of the plinian eruption from the large VFZ-volcano, Oraefajokull, in 1362 AD [4]. Glass, mineral and bulk tephra analyses show no chemical variation exceeding the analytical precision for the entire erupted volume of 2 km3 DRE. This applies even to the glass shards from distant locations in Greenland, Norway and Ireland. The total phenocryst content is 0.5-1 wt percent, with oligoclase (An14 Ab81 Or5.5), fayalite (Fa99.7 Fo0.3) and hedenbergite (Wo44.7 En2.6 Fs52.7) constituting 50- 80, 10-25 and 10-25 percent of the total phenocrysts, respectively. The extreme mineral compositions (especially pure fayalite and hedenbergite) resemble those of the granophyres in the Skaergaard and Bushveld complexes and differ from all other investigated rhyolites. The advanced fractionation and homogenisation to form the erupted 2 km3 DRE rhyolite is petrogenetically challenging, and a parental magma chamber of 20-40 km3 seems like a conservative estimate. The time-scale of the historic magma chamber evolution under Oraefajokull is limited by the large 1362-eruption, followed by a minor benmoritic eruption in 1727 and the lack of geophysical indications of present crustal magma reservoirs. [1] H Nicholson, M Condomines, JG Fitton, AE Fallick, K Gronvold, G Rogers J. Petrol. 32, 1005-1020, 1991. [2] O Sigmarsson, M Condomines, S Fourcade, Earth Planet Sci Lett 110, 149-162, 1992. [3] T Prestvik, S Goldberg, H Karlsson, K Gronvold, Earth Planet Sci Lett 190, 211–220. [4] RS Selbekk, RG Trønnes, J Volc Geotherm Res 160, 42-58, 2007.
DE: 1036 Magma chamber processes (3618)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting